1. Field of the Invention
The invention relates to a magnetometer, comprising a SQUID and a circuit arrangement for operating the SQUID, which circuit arrangement includes a coupling-in coil in a damped measuring circuit and at least one resistor, and also connection leads and contact pads whereto the SQUID, a gradiometer and electronic control circuitry are connected.
2. Description of the Related Art
Magnetometers of this kind are known. For example, from the article "Principles and Applications of SQUIDs", J. Clarke Proc. IEEE Vol. 77, No. 8, p. 1208 (1989), the preferred arrangement of a DC-SQUID for biomagnetic sensors is known. In this arrangement, a measuring circuit is associated with a SQUID ring consisting of a superconducting SQUID inductance with Josephson contacts. The measuring circuit comprises a coupling-in coil which is ultimately connected, via connection wires, to a gradiometer which serves as a measuring coil. The measuring circuit, consisting of the coupling-in coil with connection leads and also the gradiometer, must be superconducting. An external magnetic field, or more specifically its gradient, causes a current in the gradiometer which ultimately generates a magnetic flux in the SQUID ring, via the coupling-in coil. In order to enable said external magnetic flux to be coupled into the SQUID ring with an as high as possible efficiency, the coupling between the coupling-in coil and the SQUID ring must be as rigid as possible. A coupling factor of up to 0.9 can be achieved between the coupling-in coil and the SQUID ring. In order to damp parasitic effects which would cause degradation of the sinusoidal SQUID characteristic, a resistor is connected parallel to the coupling-in coil in the simplest case.
In order to linearize the output signal and to achieve adequate measuring dynamics, a SQUID is operated in a so-called flux-locked-loop circuit. This is to be understood to mean an integrating control circuit which ensures that, despite external fields, the flux in the SQUID ring remains at the point of highest sensitivity. In order to achieve this, according to the prior art the output current of the controller is fed back to a modulation coil magnetically associated with the SQUID ring. This current is then also proportional to the external magnetic field to be measured. The coupling factor between this modulation coil and the SQUID ring per se can be chosen at random and typically amounts to 0.5. The input signal for the electronic control circuit is derived from two contact pads (contact points) which are connected to the SQUID ring via connection leads. At the same time a bias current is applied to this current path in order to adjust the optimum working point.
Further embodiments of such circuit arrangements for magnetometers are known, for example from U.S. Pat. No. 4,389,612, or German Patent Application P 39 26 917. The circuit arrangements disclosed therein relate to an implementation on a DC basis or an implementation utilizing an alternating bias current. These applications also require a modulation current of corresponding frequency. It is superposed on the controller current and hence fed into the modulation coil.
Also to be mentioned is the Abstract JP-A-2-29 8878 which, however, relates to an arrangement in which the coupling-in coil is opened and fed out in order to insert additional coils. The aim is to increase the dynamic range of the system; however, this makes sense only for so-called digital SQUIDs and is of no significance for the conventional SQUIDs considered herein.
Inductive coupling is also possible for applications as known from the article "Elimination of flux-transformer crosstalk in multi-channel SQUID magnetometers", H. J. M. ter Brake, F. H. Fleuren, J. A. Ulfman, J. Flokstra, Cryogenics, Vol. 26, pp. 667-670 (1986), where absolute non-interaction is required, i.e. where not only the flux in the SQUID but also the current produced by the external field in the gradiometer must be compensated to zero.
In all these applications the SQUID (or the SQUID ting), the coupling-in coil and the modulation coil etc are often integrated on a single chip, thus forming a SQUID module. The manufacture of such a complete circuit arrangement, however, is a difficult and complex technological process where each additional manufacturing step reduces the yield. Therefore, the modulation coil is often constructed separately so as to be mounted on the actual SQUID module only at a later stage. Apart from the additional effort then required, increased tolerances occur in respect of the coupling factor between the SQUID ring and the modulation coil. However, because this coupling factor has a direct effect on the circuit gain of the controller, the adjustment range of the latter must be increased accordingly.
A further step required according to the state of the art is imposed by the possibility of freezing of the magnetic flux of a SQUID module. This is because it often occurs that SQUIDs, because of their superconducting structures, freeze magnetic fluxes which are caused by external disturbances of an electric and/or magnetic nature, ultimately resulting in degeneration of their characteristic. Relief in this respect is only provided by heating, i.e. briefly exceeding the transition temperature of the superconducting materials used. From U.S. Pat. No. 4,689,559, it is known to arrange a heating resistor in the vicinity of the SQUID (or the SQUID ring), which resistor need receive comparatively large current and voltage values via additional connection leads.